Ortho-alkylated hydroxyaromatic compounds are useful for a variety of purposes. For example, ortho-cresol is a useful disinfectant and wood preservative. It is often prepared by the vapor-phase reaction of a phenol with methanol. In another alkylation reaction, ortho-cresol and phenol can both be converted into 2,6-xylenol. This xylenol monomer can be polymerized to form poly(2,6-dimethyl-1,4-phenylene)ether, which is the primary component in certain high-performance thermoplastic products.
The alkylated hydroxyaromatic compounds are usually prepared by the alkylation of the precursor hydroxyaromatic compound with a primary or secondary alcohol. The alkylation must be carried out in the presence of a suitable catalyst, such as a magnesium-based compound. U.S. Pat. Nos. 4,554,267; 4,201,880; and 3,446,856 describe the use of magnesium oxide for this purpose.
A great deal of attention has been paid to optimizing the performance of magnesium-based catalysts in an industrial setting. Usually, it is very important for the catalyst to have high activity, i.e., it must have as long an active life as possible. Moreover, the catalyst must have very good ortho-selectivity. Many of the ortho-alkylation catalysts used in the past produced a high proportion of para-alkylated products of marginal utility.
As an illustration, the alkylation of phenol with methanol in the presence of a magnesium oxide catalyst yields ortho-cresol (o-cresol) and 2,6-xylenol, which are desirable products. However, the alkylation reaction may also produce substantial amounts of para-substituted compounds, such as para-cresol (p-cresol); 2,4-xylenol, and mesitol (2,4,6-trimethylphenol). In some end use applications, these para-substituted compounds are much less useful than the corresponding compounds containing unsubstituted para positions. For example, polyphenylene ethers prepared from such compounds lack the desired properties obtained when the starting material is primarily 2,6-xylenol.
Selectivity and activity are related to the characteristics of the ortho-alkylation catalyst, and to the manner in which it is prepared. In the above-mentioned U.S. Pat. No. 4,554,267 (Chambers et al), a magnesium-based catalyst is prepared with a slurry process, using selected amounts of a copper salt as a promoter. In the process, the magnesium reagent and an aqueous solution of the copper salt are combined to form a magnesium-containing solid phase, which includes uniform, well-dispersed copper. The solid phase is dried, shaped, and calcined. The catalyst system is then used in the alkylation reaction of phenol and methanol. The reaction produces relatively high levels of the desirable 2,6-xylenol product. Moreover, the "selectivity" of the catalyst system, i.e., the ratio of 2,6-xylenol yield to the combined yield of 2,4-xylenol and mesitol, is also quite high, as is the overall yield of 2,6-xylenol.
It is clear that a catalyst composition like that described in the patent of Chambers et al is very useful and effective for alkylation reactions. Moreover, the slurry process used to prepare such a catalyst can be efficiently carried out in some situations. However, there are drawbacks associated with the slurry process in other situations--especially in a large-scale production setting. For example, the "liquid"-related steps, which involve pre-blending of a copper compound with a magnesium compound, usually require mixing and holding tanks, recirculation piping, and specialized drying systems. Storage of the dried magnesium oxide/copper product (sometimes referred to as a "matrix") may also be required, prior to blending and shaping steps. These operations and the related equipment represent a considerable investment in time and expense (e.g., energy costs), and may therefore lower productivity in a commercial venue. Furthermore, use of the slurry process can sometimes introduce metal and halogen-based contaminants into the catalyst, via the water supply.
It should therefore be apparent that improved methods for alkylating hydroxyaromatic compounds would be welcome in the art. The improvements may advantageously depend on the catalyst systems used in the alkylation reaction. Thus, enhanced techniques for preparing the catalyst would also be very desirable. Any new process related to alkylation or catalyst preparation should provide significant advantages in one or more of the following aspects: catalyst selectivity, catalyst activity, product yield, cost savings, and overall productivity. Moreover, use of the new processes should result in products (e.g., 2,6-xylenol) which possess substantially all of the desirable characteristics of products made by prior art methods.